The present invention relates to light-emitting diodes.
For the purposes of the present discussion, the present invention will be discussed in terms of a xe2x80x9cwhitexe2x80x9d emitting light-emitting diode (LED); however, the methods taught in the present invention can be applied to wide range of LEDs. A white emitting LED that emits light that is perceived by a human observer to be xe2x80x9cwhitexe2x80x9d can be constructed by making a LED that emits a combination of blue and yellow light in the proper ratio of intensities. High intensity blue-emitting LEDs are known to the art. Yellow light can be generated from the blue light by converting some of the blue photons via an appropriate phosphor. In one design, a transparent layer containing dispersed particles of the phosphor covers a LED chip. The phosphor particles are dispersed in a potting material that surrounds the light-emitting surfaces of the blue LED. To obtain a white emitting LED, the thickness and uniformity of the dispersed phosphor particles must be tightly controlled.
In one design, the LED is mounted on a heat sink in a well in a printed circuit board base. The well has reflective sides that form a reflective xe2x80x9ccupxe2x80x9d having the LED chip at the bottom thereof. The phosphor is mixed with a liquid casting epoxy and injected into the cup. The part is then heat-cured for 2 hours.
Unfortunately, this manufacturing system has a poor yield due to uneven phosphor dispersion in the reflecting cup. The density of the phosphor particles is larger than that of the liquid casting epoxy, and hence, the particles tend to settle toward the bottom of the reflector cup. As a result, the amount of phosphor over the chip is reduced, which, in turn, lowers the ratio of yellow to blue light generated by the completed device. Such a device emits light that is bluish-white rather than white.
In addition, the liquid casting epoxy tends to shrink during the heat curing process. This can leave a part in which the top of the chip is exposed. This also leads to a color shift that is undesirable.
The present invention is a LED that includes a light emitter that emits light of a first wavelength and a phosphor layer that converts a portion of that light to light of a second wavelength. The phosphor layer includes a powdered phosphor suspended in a photo-curable medium that sets upon exposure to light of a curing wavelength. The phosphor layer can also include a thixotropic agent that reduces the rate at which the powdered phosphor settles in the medium prior to the medium being exposed to light of the curing wavelength. The photo-curable medium preferably includes a photo-curable epoxy that cures in a time that is short compared to the settling time of the phosphor powder in the medium.